1. Field of the Invention
The present invention broadly relates to circulation controlled airfoils and, more particularly, is concerned with the provision of multiple blowing jet slots at the blunt trailing edge surface of an airfoil such that the location of the jet slots relative to one another can be varied to provide optimized blowing for given flight conditions.
2. Description of the Prior Art
During recent years, there has been considerable interest in circulation control techniques that improve the short take-off and landing capabilities of fixed wing aircraft. There also has been interest in applying circulation control to helicopter rotors. One of the more promising circulation control techniques takes advantage of the Coanda effect, which allows a high speed jet to attach to and flow around a curved surface. This effect is primarily due to a balance between the centrifugal forces in the jet and the reduced pressure on a curved surface. Circulation control airfoils take advantage of this effect by introducing a jet of high energy air into the suction surface trailing edge region of a blunt-edged airfoil enabling the flow to remain attached for a greater distance before separation. Specifically, a two-dimensional wall jet slot is used to force the rear stagnation point down onto the lower surface of the airfoil with a corresponding increase in circulation and thereby lift. Airflow emerges from the jet slot in the form of a sheet that flows onto and around the surface of the trailing edge (due to Coanda effect). The air adheres to the rounded surface and then separates downward off the trailing edge.
Some advantages of circulation control become apparent when comparisons are made with conventional airfoil designs. For example, conventional airfoils with mechanical flaps usually cannot produce the high lift coefficients possible with blown airfoils due to the onset of flow separation. Also, blown airfoils are able to vary lift simply by varying the blowing rate.
Research has shown that the lift of the airfoil for a given blowing coefficient can be improved by using more than one blowing jet slot. To increase the lift over that obtained with single jet blowing, the dual jet slots must be positioned advantageously in the trailing edge region of the airfoil to prevent flow separation and enable the movement of the stagnation points. U.S. Pat. No. 3,873,233 to Linck discloses a circulation controlled airfoil having a pair of blowing jet slots along and tangential to the blunt trailing edge surface of the airfoil. Kind and Maull, in "An Experimental Investigation of a Low-Speed Circulation-Controlled Aerofoil," The Aeronautical Quarterly, Vol. 19, May 1968, pages 170-182, reported on results using two jet slots in the trailing edge region of the airfoil; however, their jet slots opposed each other, and flow from the second jet slot reduced lift. Also, R. V. Smith, in "A Theoretical and Experimental Study of Circulation Control with reference to Fixed Wing Applications, " Research Paper No. 582, University of Southampton, U.K., July 1978, reported on multiple jet slots which could be rotated together around the trailing edge, but were fixed relative to each other.
Advantageously located multiple jet slots would appear to provide an effective means for increasing the lift coefficient; however, it is perceived that there exists a need for improvements being made thereto in order to enhance their adaptability to varying flight conditions.